Use en-US as fallback when using other default language (#21200)
Only en-US has complete translations. When use other language as
default, the en-US should still be used as fallback.
Close #21199
### Screenshot
![image](https://user-images.githubusercontent.com/2114189/190882906-b7a83958-0ea2-46c4-9084-42c4f9a239aa.png)
Co-authored-by: Lauris BH <lauris@nix.lv>